Studies completed, under way or planned include: a) in vivo studies of the effect of drug metabolism inhibitors or inducers on the metabolism, distribution, covalent binding and nephrotoxicity of MeCCNU in F344 rats, b) in vitro studies on the metabolism and covalent binding of MeCCNU by rat liver or kidney microsomes, and c) studies on the role of endogenous GSH as a protective factor in modulating the nephrotoxicity of McCCNU. These studies demonstrate that reactive metabolites and/or degradation products of MeCCNU are accumulated preferentially in kidney and that pretreatment of rats with an inhibitor of cytochrome P-450, piperonyl butoxide, decreased the metabolism and covalent binding of MeCCNU to liver and kidney macromolecules and ameliorated MeCCNU nephrotoxicity. Furthermore, rat liver, but not kidney, microsomes catalyzed the alkylation of chloroethyl-derived MeCCNU to proteins by a reaction that was both oxygen and NADPH dependent, and was inhibited by the addition of either piperonyl butoxide or glutathione to the reaction medium. In contrast, rat liver microsomes metabolized the cyclohexyl moiety of MeCCNU to products with less carbamylating activity than as observed by the non-enzymatic degradation of MeCCNU. Endogenous glutathione appears to play a major protective role against MeCCNU toxicity in the kidney.